Films formed from thermoplastic polymers, such as polyolefins, polyesters or polyamides are known as films intended for medicine, hygienic materials, common household materials and industrial materials, and especially moisture-permeable and waterproof films.
For example, PTL 1 discloses a method for producing a moisture-permeable and waterproof film which is a method for producing a porous film in which a resin composition comprising a polyolefin resin and an inorganic filler is formed into a film, and the obtained film is stretched by gear stretching to a factor of 1.2 to 3 in the direction perpendicular to the machine direction, to open pores.
However, the thermoplastic polymers mentioned as olefin resins in PTL 1 are chemically stable in ordinary environments, or in other words they have low biodegradability, and therefore the thermoplastic polymers are problematic in terms of their resistance to decomposition in soil. In addition, since the aforementioned thermoplastic polymers are produced from crude oil-derived materials, i.e. they have low biomass, the thermoplastic polymers release large amounts of carbon dioxide during incineration, and are therefore problematic from the viewpoint of environmental protection.
In addition, in the method for producing a porous film disclosed in PTL 1, the film before gear stretching must have a uniform film thickness and excellent flatness, as well as no reduction in strength or ductility in the cross direction perpendicular to the machine direction, while gear stretching is only possible in the cross direction perpendicular to the machine direction. The method described in PTL 1 allows uniaxial stretching in the machine direction.
Therefore, gear stretching of the porous film of PTL 1 does not produce flexibility in the machine direction, even though it produces flexibility in the cross direction that is perpendicular to the machine direction. In addition, since uniaxial stretching of the porous film of PTL 1 in the machine direction produces stiffness in the machine direction, this causes a problem in that the properties in the machine direction differ significantly from the properties in the cross direction which is perpendicular to the machine direction.
Biodegradable moisture-permeable and waterproof films are also known, such as the uniaxially stretched porous film comprising polylactic acid and a fine powdered filler, disclosed in PTL 2. However, polylactic acid is generally harder than polyolefin resins, while also having higher elasticity and lower breaking elongation, and therefore flexibility cannot easily be imparted to its films by uniaxial stretching. The porous film disclosed in PTL 2 therefore has much room for improvement in terms of flexibility.